Firstly, im going to start off with a little education with the difference between theory and scientific theory.  While having a theory is your idea about something or a hypothesis, a scientific theory is a hypothesis that has gone through rigorous testing, tens of thousands if not millions of peer reviewed studies and experiments and is accepted worldwide as fact(ex. gravity, big bang, theory of relativity). 
 Now I know I just lost at least three of you to go comment how I can call the big bang "Fact", but let me break it down nice and simple for everyone.  As we see through spectrography(an instrument that separates an incoming wave into a frequency spectrum) every other galaxy we see in the universe, aside from Andromeda, is moving away from us.  We KNOW this by using The Doppler Effect.  When we see that galaxies are giving off a faint red hue, the waves frequency is being stretched showing that they are traveling away from us.  On the contrary, if they gave off a blue color the frequency of the wave is being compressed showing the object is coming towards us.  Now with our new found knowledge of The Doppler Effect when you see that every single galaxy in our universe is flying away from each other at tremendous speeds what happens if you turn back the clock?  Well now if you are recording a person walking south down a street then you hit rewind on your camera what way is that person traveling? NORTH! correct!  Now lets combine this logic with our new knowledge of The Doppler Effect and what would you say would happen to all of the galaxies?!?  They all would eventually come back into a singular point between 13.7 and 14.6 billion years ago.  There you go you just learned today why the big bang is a widely accepted scientific theory. 

Common arguments of the ignorant: 1) Its only a theory and everyone knows theories are just guesses...2)Everything in space is speculation and guess(I mean if you call observing, sampling, and testing guessing then sure).....3)God did it...
 Tune in next time for light speed, black holes, and greenhouse planets.  Feel free to ask any and all space related questions, or hell even earth bound science questions the choice is yours.  Cheers and have a great day.